In the construction of motor vehicles the safety of the motor vehicle and economy of production and operation both have important roles to play. On the one hand the body or the chassis of the motor vehicle should provide a high level of safety in a crash, and on the other the weight of these components should be kept as low as possible in order to lower material costs and operating costs. For this reason in the state of the art hardened components, preferably hot formed or press cured components are used. To this end sheet steel or a pre-formed component is heated to an austenetisation temperature of higher than AC3 and then rapidly cooled in a tool, so that within the component a martensitic and/or a bainitic structure develops. In this way strengths Rm of 1200-1600 MPa, yield strengths Rp0.2 of more than 900 MPa and A80 elongation at break values of up to 6% can be achieved. Such components have high dimensional stability and are highly resistant to deformation in a crash. But these components do lack residual strain capability. In order to avoid cracking of the components due to their high level of hardness, it is necessary that the components also have a certain ductility. In order to achieve this, such components are tempered following a press curing or hot forming process. Up until now during such tempering processes the components have been tempered for a dwell time of, for example, approximately 10 minutes at an average temperature of 400° C. The components tempered in this way demonstrate a clear improvement in their ductility or their folding behaviour. In order to reduce the risk of material failure during an axial crash loading, i.e. in particular during a head-on crash or rear shunt, it is necessary, however, to increase the elongation at break values A80 of the components.
Elongation at break means the residual relative change in length compared with the starting length after the break of the test piece in a tensile test. Here the elongation at break value A5 relates to a round test piece, the starting length of which is five times its diameter. The elongation at break value A80 on the other hand refers to a test piece with a starting length of 80 mm. For the same A5 material the elongation at break value will take higher values than the elongation at break values A80. Unless otherwise stated, in this application the elongation at break value A80 is intended.
From DE 10 2005 054 847 B3 a highly rigid steel component is known for which the elongation at break value A5 was increased by a tempering process in the temperature range between 320 and 400° C. to between 6% and 12%. It has been shown, however, that the known method does not lead to high elongation at break values with sufficient reliability.